2018 Mazda CX-9: What to check before buying

Cracked cylinder heads that leak coolant are the dominant problem. Some leaks reach the engine oil. Warning signs include coolant puddles under the parked car, an empty reservoir, rough idle, stalling, bucking when coming to a stop, white smoke, and overheating on the highway. Many owners get no warning light before the engine is damaged, and several cars fail while driving and need a tow. Reported failures range from about 50,000 to 135,000 miles. Dealers usually recommend a full engine replacement, with quotes near $11,000. Owners say Mazda often refuses help once the car is past its warranty.

Coolant leaks into the cylinders through a fractured cylinder head, and dealers recommend replacing the engine. Symptoms include an overheating message and a stop-driving warning within minutes of setting off, an unusual smell inside the cabin, and a coolant trail under the parked car. One failure is logged at about 88,080 miles. Neither reported vehicle had been repaired at the time of filing.

Spider cracking of the infotainment screen makes up the largest share. Cracks spread from the corners and hide the camera and navigation images, sometimes on cars with low miles. One fire report says a passenger seat heater shorted, smoldered, and burned the seat. Another owner was told the engine wiring harness was defective and a fire risk. Other reports include Smart Brake System warnings followed by the car braking on its own at highway speed, a liftgate that opens by itself and drains the battery, sudden dead batteries, and false key and ignition warnings.

Coolant leaks at the cylinder head near the exhaust manifold dominate this category. Several owners say they saw no warning lamp or temperature change. The leak shows up as fluid dripping onto the floor, a sloshing sound, or a radiator fan that keeps running after shutdown. In some cases the damage spreads to the exhaust manifold, the turbocharger, or the catalytic converter. Reported mileages range from about 53,300 to 102,000. A head gasket and resurfacing job did not hold in one case. Separately, one owner reports the forward camera failing along with smart brake warnings and rough running.

The most serious report involves the electronic parking brake. It either refuses to release, stranding the driver on an incline, or lets go suddenly while the car is in gear, and it is tied to a crash. Another owner says the pedal went hard with no give and the car rolled over a curb into a railing. One family says the SCBS light came on and the car slowed to a crawl on the road without brake lights, nearly causing a rear-end collision. Squealing and vibration before 25,000 miles that came back after the rotors were resurfaced also appear.

Knocking noises start suddenly when the wheel is turned, with no damage to explain them. A mechanic cited a service bulletin, but no recall applied. The other two reports in this category describe different problems. One covers harsh, slipping gear changes, lunging forward at stops, and a water pump replaced for a failing bearing. The other covers a cluster of keyless entry, charging, overheating, and battery warnings on a car with very low miles, at one point while it was moving with a family aboard.

Both reports involve crashes in which the front airbags did not deploy, and an occupant was injured in each one. One was a head-on collision with several cars where every other vehicle's airbags went off. The other driver also describes repeated front windshield error messages that make the car stop automatically on freeways and streets. That driver says three dealer visits produced a different explanation each time.

A passenger heated seat overheats while driving, fills the cabin with a burning smell and heavy smoke, and burns a hole through the upholstery. Turning the heater off stopped the smoke. Separately, the rear left seat rails stop latching without warning, so the seat could slide forward in a crash. That is the most common spot for a child seat. A mechanic reports Mazda sells only a complete replacement seat, at about $5,000.

Only two problems appear here. The passenger sun visor will not lock in the up position and droops into the line of sight. The sunroof glass shatters while the car is moving. Neither report includes a mileage or the cause of the failure.